Microwave switching device



Sept. 16, 1969 o m ET AL MICROWAVE SWITCHING DEVICE Filed Sept. 2, 1966INVENTORS HARRY GOLDIE 8| [3 ATTORNEY THEODORE M. N E LSON WITNESSES WWW%WJ@ United States Patent US. Cl. 31539 3 Claims ABSTRACT OF THEDISCLOSURE A microwave switching device in the form of a resonantdischarge device in which a tuning post of a resonant iris cooperateswith a sidewall of a waveguide to constitute a discharge gap within thewaveguide. A keep-alive electrode, located in a chamber external to thewaveguide, is positioned in alignment with the tuning post and anaperture in the waveguide.

This invention relates generally to microwave frequency gaseousdischarge switching devices and more particularly to those devicesincorporating a keep-alive electrode.

In prior art microwave switching tubes, more commonly referred to as TRtubes, the devices consisted of two truncated cones geometricallyopposed within a wave guide portion or a cavity resonator. The spacebetween the two cones provided the discharge gap region in whichbreakdown resulted on the transmission of adequate microwave frequencyenergy. It is desirable to provide a keep-alive electrode structurewhich will furnish a residual supply of electrons adjacent to thisdischarge gap region to ensure reliable operation; that is to obtain aconsistent breakdown level with each incident radio frequency pulse.

In the prior art design utilizing the two cone electrodes, the mostcommon practice was to provide an internal keep-alive electrode within ahollowed out portion of one of the cone electrodes. The geometricalconstraints on the design of such a cone electrode including thekeepalive electrode provide a multitude of problems. For example, aninsulating sleeve is normally provided between the hollowed out portionof the cone and the keepalive electrode. The insulating sleeve wastherefore in intimate contact with the negative keep-alive electrode andalso in the region adjacent to the discharge gap. The close proximity ofthe insulator to the keep-alive electrode is found to be one of thesignificant reasons for short lifetime in TR devices. This comes aboutbecause the heavy positive ions are accelerated toward the negativekeep-alive electrode, bombarding it and causing the liberation ofmetallic particles. These metallic particles diffuse toward theinsulating sleeve and coat it with the metallic deposit. This coatingcauses a continuous and slow change in the static field distributionbetween the cone tip and the negative discharge electrode. This leads atfirst to unreliable radio frequency gap priming with consequentfiuxation in the leakage characteristics, and ultimately to a completeshort circuit between the keepalive electrode and the cone electrode.

It is accordingly the principal object of this invention to provide animproved switching device to overcome the problems of the prior art.

It is another object to provide an improved switching device in which animproved keep-alive electrode system is provided.

It is another object to provide an improved keep-alive electrode systemfor a switching device in which bom- 3,467,883 Patented Sept. 16, 1969bardment of the keep-alive electrode by positive ions does not reducethe lifetime of the device.

'It is still another object of the invention to provide an improvedkeep-alive electrode system for a switching device in which theinsulating member holding the keepalive electrode from the switchingdevice is remote from the discharge region.

It is still another object to provide an improved baflle structure for asingle cone device.

Briefly, the present invention accomplishes the abovecited objects byproviding a microwave switching device utilizing a single coneelectrode, full height wave guide with an aperture provided in the wallof the wave guide opposite the apex of the single cone electrode. Thekeepalive electrode is positioned exterior of the aperture and supportedwithin a tubular metallic member with an in sulating sealing memberremote from the aperture positioning and retaining the metallicelectrode within the tubular member and providing a long paththerebetween. An improved baffle is also provided to obtain tuning inconjunction with the single cone.

These and other objects and advantages of the present invention willbecome more apparent when considered in view of the following detaileddescription and drawings, in which:

FIGURE 1 is a perspective view of an embodiment of the invention with aportion of the switching device broken away to reveal internalstructure; and

FIG. 2 is an enlarged sectional view of the embodiment shown in FIG. 1taken along the line II-II.

Referring now to the drawings, the embodiment shown in FIGS. 1 and 2 isa TR switching tube comprising a section of a rectangular wave guide 2of good electrical conductive material such as copper defining parallelbroad upper and lower walls 4 and 6 and narrow side walls 8 and 10.Secured adjacent the ends of the wave guide 2 are conventional matingflanges 12 and 14. The flanges 12 and 14 are utilized to secure theswitching device to suitable microwave energy wave guides. A resonantwindow assembly 16 closes each end of the wave guide section 2 toprovide a hermetically sealed envelope 3. Each window assembly 16includes a dielectric member transmissive to microwave energy. Theenvelope 3 defined by the wave guide section 2 and window assemblies 16contains a gaseous ionizable atmosphere such as argon gas or water vaporat reduced pressure such as 10 torrs to provide a high intense gaseousdischarge when high power microwave frequency energy is transmittedtherethrough. In the case of an X-band switching device the dimensionsof the rectangular wave' guide 2 are 1 inch by /2 inch.

Positioned within the wave guide 2 is a baffle 21 which includes twosymmetrical electrically conductive vanes 22 and 24 transverse to thewave guide 2. Mounted between the inner edges of the vanes 22 and 24 isa tubular member 33 of a suitable electrically conductive material suchas copper. The tubular member 33 is secured to the bottom wall 6 andextends about /5 of the distance to the upper wall 4. The inner edges ofthe vanes 22 and 24 are secured to the outer surface of the tubularmember 33. The edges of the vanes 22 and 24 between the member 33 andthe upper wall 4 are recessed back so as to provide an iris 31.

An electrode 26, having a cylindrical cross-section, extends through thelower wall 6 of the wave guide 2 and thorugh the tubular member 33 tothe iris 31. The elec trode 26 is in sliding engagement with the member33. The lectrode 26 includes a pointed portion 28 with the apex 29 ofthe pointed portion 28 adjacent the wall 4. The pointed portion 28 ofthe electrode 26 is within the iris 31 and constitutes a tuning post forthe latter. The

electrode 26 is of a suitable electrically conductive material such asKovar alloy. The electrode 26 is positioned and retained within the waveguide section 2 by means of an annular member 32 which is of a suitablematerial such as nickel which is brazed to a seat portion in the outersurface of the wall 6. A flexible diaphragm 34 which is also annular inshape and of a suitable material such as copper has its outer peripherybrazed to the member 32 and its inner periphery to the electrode 26. Thediaphragm 34 hermetically seals the electrode 26 within the wave guide 2and also allows movement of the electrode 26 within the member 33 so asto provide the proper space for a discharge gap. The spacing of the apex29 of the pointed portion 28 of the electrode 26 from the wall 4 mayvary considerably. An aperture 40 is provided in the wall 4 opposite theapex 29 of the pointed portion 28. The diameter of the aperture 40 isabout 20 mils.

Positioned exteriorly of the wave guide 2 and secured to the wall 4 isthe keep-alive electrode system consisting of a tubular member 42 of asuitable material such as Kovar alloy which is secured to the outersurface of the wall 4 by brazing. A second tubular member 44 is in turnsecured to the opposite end of the member 42. The tubular member 44 isof a suitable material such as Kovar alloy which is sealed to aninsulating ring seal 46 of a suitable material such as glass which inturn supports a keep-alive electrode 48. The keep-alive electrode 48 isof an electrically conductive material such as Kovar alloy and extendsperpendicularly to the wall 4 and to a point adjacent the aperture 40.The insulating material of the seal 46 extends down along the surface ofthe keep-alive electrode 48 to the end portion adjacent the aperture 40.

In the operaiton of the device, microwave energy is connected to thewave guide 2 in a well known manner and a DC voltage is applied to thekeep-alive electrode 48 such that in most cases the electrode 48 isnegative with respect to the wave guide 2. The application of thevoltage to the keep-alive 48 causes the generation of electrons whichcauses ionization of the gas in the envelope 3 in the immediate regionbetween the apex 29 of the electrode 26 and the keep-alive electrode 48.The result is that when sufficient microwave energy is impressed acrossthe space discharge gap between the apex 29 of the electrode 26 and thewall 4 to cause electrical breakdown.

A table of representative values and data obtained from the describedembodiment is shown below in which w is the width of the iris 31 betweenthe vanes 22 and 24. l indicates the height of the iris 31 between thetwo vanes 22 and 24 and the upper wall 4. The letter d indicates thevarious vertical spacings of the apex 29 of the electrode 26 from theinner surface of the wall 4. The negative value indicates that the apex29 is inside the aperture 40. Q indicates the frequency bandwidth of thecavity. VSWR indicates the reflected microwave voltage of the unfireddevice. The letter f indicates the frequency in gigahertz and Lindicates the insertion losses.

LOW LEVEL PARAMETERS OF SINGLE-STAGE SINGLE CONE TR W, millil, millid,milli- VSWR in L1, dbs. inches inches inches QL f0 gHz t.

It is found that in restricting the discharge to the region between thekeep-alive electrode 48 and wall of the aperture 40 where no insulationexists and providing a. long insulating path from the exposed portion ofthe electrode 48 to the member 44 that the lifetime of the device issubstantially extended. Portions of the insulator 46 are also remotefrom the discharge and the products of sputtering will difiuse to themetallic surfaces of the wall 4, tubular member 42 and tubular member 44where .4 there will not be an adverse effect. This design leaves all ofthe critical DC gap lengths and electrode dimensions of the keep-aliveunchanged. The essential parameters, consistent with effective TRdesign, is to obtain low Q, simultaneously with small R.F. gap lengths.The small R.F. gap lengths are essential for low firing power which is aprerequisite to low leakage power characteristics. Extensive low levelmeasurements as indicated above were made to obtain families of curvesto provide a wide latitude in design.

Other advantages of the structure are that the tuning diaphragm does notsee any of the R.F. currents and that the very small aperture leads tohigh values of fired isolation. The bafile 21 enables the building of asingle cone full height resonant element stage of low Q and widebandwidth.

Various modifications may be made within the spirit of the invention.For example, for a fixed tuned TR tube the vanes 22 and 24 could besecured to the electrode 26 and the tubular member 33 would not berequired.

We claim:

1. A microwave switching device in the form of a resonant type dischargedevice comprising a section of waveguide; a conducting baffie plateextending transversely of said waveguide to constitute a transverse irisplate having an iris notch in one edge thereof adjacent one wall of saidwaveguide, the remaining edges of said baffie plate being in ohmiccontact with the walls of said waveguide; an aperture in said wallintermediate the sides of said notch; a tuning post in said notchadjustable toward and away from said aperture; a keepalive electrodeadjacent said aperture but on the side of the waveguide wall oppositethat of said tuning post for supplying free electrons in the volumebetween said tuning post and the edges of said aperture; an auxiliarychamber formed of a cylindrical member of electricallyconductingmaterial disposed exteriorally of said waveguide encompassing saidaperture and having one of its edges ohmically connected to the wall ofsaid waveguide; and insulation means carried by the other end of saidcylindrical member and supporting said keep-alive electrode, wherebysaid insulation means is removed from the vicinity of the plasmadischarge region around said aperture and tuning post.

2. The microwave switching device as set forth in claim 1 in which saidtuning post is substantially circular in cross-section with a pointedend portion adjacent said aperture.

3. The microwave switching device as set forth in claim 1 in which saidkeep-alive electrode is a rod positioned substantially perpendicular tosaid waveguide wall, said insulating means is annular in shape toprovide a seal between said keep-alive electrode and said cylindricalmember, and said seal is positioned a greater distance from saidaperture than the distance from said keepalive electrode to saidcylindical member.

References Cited UNITED STATES PATENTS 2,611,109 9/1952 Casellini 333-13XR 2,697,800 12/1954 Roberts 3 l5-39 2,903,623 9/1959 Walker 33313 XR3,310,706 3/1967 Woermbke 33313 XR OTHER REFERENCES Microwave Duplexers,Smullin and Montgomery,

McGraw-Hill, 1948, Tk 6590, M49, S5. Pages 69, 70, 107 and 109 reliedupon.

HERMAN K. SAALBACH, Primary Examiner M. NUSSBAUM, Assistant Examiner US.Cl. X.R. 333-13

